The present invention relates to an expandable intraluminal endoprosthesis comprising a tubular member having a first and second end and a wall surface disposed between said first and second end, the wall having a first diameter in a first, unexpanded state which permits intraluminal delivery of the member into a lumen of a body passageway, particularly a blood vessel, and being capable of acquiring a second diameter in an expanded and deformed state upon the exertion of a radially outwardly extending force to expand the lumen of the body passageway. More particularly the invention relates to an expandable intraluminal vascular endoprosthesis which is especially useful for repairing or reconstructing blood vessels narrowed or occluded by a disease. Commonly this kind of medical device is referred to as vascular stent or graft.
Stents are prosthetic devices which are implanted inside a lumen in order to provide support for its wall and to assure an undisturbed flow through the lumen. This is particularly important in the field of angioplasty which is concerned with the repair and reconstruction of blood vessels. In that particular field stents are implanted within the vascular system to reinforce collapsing. partially occluded, weakened, or abnormally dilated sections of blood vessels. More generally, however, stents can be used inside the lumen of any physiological conduit or duct including the arteries, veins, bile ducts, the urinary tract, alimentary tracts, the tracheobronchial tree, a cerebral aqueduct and the genitourinary system. Moreover stents can be used inside lumina of animals besides humans.
Generally two types of stents may be distinguished. First there are self-expandable stents which automatically expand once they are released to assume a permanent deployed, expanded state. The outwardly extending force necessary for its deployment is provided by the spring force of the material used and is accordingly inherently available within the device itself. These stents expand to a defined diameter but are unable to remodel the true vascular anatomy over lengths greater than 2 cm. Their drawback is that the physician needs to place the right device and thereby has to rely on information derived from fluoro and angiographic equipment. However, the same spring force which is responsible for the deployment of the device offers it a relatively large hoop strength, i.e. the ability to withstand radial forces which are exerted on it from the outside.
A second type of stent concerns the so-called balloon expandable stent which generally involves a tubular member capable of receiving a balloon of a balloon-tipped catheter by means of which it may be deployed. A common procedure for implanting a balloon-expandable stent in a blood vessel involves mounting the stent in its unexpanded, crimped state on a balloon-tipped catheter of a suitable delivery system, The catheter is then slipped through an incision in the vessel wall and down the length of the vessel until it is positioned to bridge the diseased or narrowed portion of the vessel. The stent is then expanded with the aid of the balloon catheter against the internal wall of the vessel, This may be done after the vessel has been predilated and it has been determined that a stent is necessary. Alternatively the vessel could be dilated by the stent itself while the latter is expanded by means of the balloon. It both cases the stent will maintain its deployed, expanded form once the balloon is evacuated and the catheter retracted again in order to provide a permanent support for the blood vessel concerned.
A wide overview of vascular stents which are nowadays available is given in the Handbook of Coronary Stents by Patrick W. Serruys et al. of the Rotterdam Thoraxcentre Interventional Cardiology Group. This overview describes at page 21 ff. the so called Palmaz-Schatz(trademark) stent as the gold standard in the field of stents. This stent concerns a number of consecutive slotted tubes of stainless steel which are mutually connected by means of one or more brides. Although this stent is most widely used and tested in practice, having been implanted in over 600000 patients all over the world, it still suffers from a number of drawbacks. The main drawbacks have to do with the stent-to-vessel-ratio uniformity and crimped as well as deployed flexibility. The stent-to-vessel-ratio involves the degree to which the vessel is supported by the stent in its expanded state and should not only be high, but preferably also uniform throughout the length of the stent. However, due to the inevitable bridges between adjacent tubes of the Palmaz-Schatz(trademark) stent, there will be a bare area between adjacent segments of the stent once it has been deployed, giving rise to a decreased and even poor stent-to-vessel-ratio at these locations. The other drawback concerns the rather high rigidity of the stent segments in their crimped and deployed state. As a consequence, the stent has only a limited flexibility which hinders the delivery of the stent to its intended position inside the body. The poor deployed flexibility of this stent gives rise to a straightening of the vessel over segments longer than typically 2 cm which appears to be a primary cause for late term restenosis of the stented area. Typically this may occur about 6 months after the stent implantation.
A balloon expandable stent with a highly uniform stent-to-vessel ratio us well as an excellent flexibility in its crimped state is described at page 63 ff. of the same reference and concerns the Cordis Coronary Stent. This device is composed of a single piece of tantalum (Ta) wire. The wire is wrapped to form a continuous sine wave and helically wound along a longitudinal axis. Both ends of the wire are weld terminated. A similar device was presented at the annual symposium of the Radiological Society of North America (RSNA) 11/95. This peripheral stent embodiment incorporates intermediate welds, patterned through the length of the stent. This device contains adjacent helical turns of the wire which are welded together at adjoining locations and exhibits a highly regular distribution of the wire along the length of the device. Its properties, such as crimped profile and stent-to-vessel ratio, are uniform over its length both in the crimped and deployed states. However, because of its constitution this device offers only a poor design freedom when it comes to tailoring the design to add specific functionality and remove certain drawbacks. Also the internal stress in the device once it has been wound hinders the provision of reliable welds between adjacent turns. Moreover, these welds as well as those to the ends of the wire, remain a weak point especially during expansion of the device.
A balloon expandable stent which combines a high degree of uniformity and flexibility with excellent design capabilities is described in the co-pending European patent application 98201446.6 by applicant. This device features a substantially continuous structure of mutually staggered undulations which has been separated from a tube wall. Said substantially continuous structure comprises at least one pattern which advances substantially helically along a longitudinal axis of said tubular body and comprises connection elements connecting adjacent undulations. The connection elements are an integral extension of the undulations which they connected.
Although the device of said co-pending application provides excellent properties especially regarding its flexibility both in a compressed and deployed state, it may still be prone to damage when subjected to an external load. As such it may be less suitable for application in peripheral vessels and arteries, for instance those in the limbs and abdominal region of a human body, which lack substantial protection by the skeleton.
It is an object of the present invention to provide an expandable intraluminal endoprosthesis of the kind referred to in the opening paragraph with an improved ability to withstand external forces such that it is suitable for application in said peripheral vessels.
To this end, an expandable intraluminal endoprosthesis of the type described in the opening paragraph is, according to the present invention, characterized in that at least a part of said wall of said tubular member comprises, at least one substantially continuous winding of mutually staggered primary undulations advancing substantially helically along a longitudinal axis of said tubular member, in that a first primary undulation is connected to an associated second primary undulation by means of a first connection element, in that a third primary undulation subsequent to said first primary undulation is connected to an associated fourth primary undulation subsequent to said second primary undulation by means of a second connection element, and in that said first and second connection elements are mutually connected by means of a connection strut which lies interposed between said primary undulations, at least in said first unexpanded state. The additional material delivered by the connection strut gives the device more rigidity in the deployed state. This is especially predominant in a preferred embodiment of the endoprosthesis which according to the invention is characterized in that primary undulations are mutually interconnected by means of a number of connection elements which are regularly distributed over a helical turn of said at least one winding and in that a connection strut is present from each connection element within said turn to a subsequent connection element within said turn. In a sense, these connection struts between connection elements provide a secondary scaffolding pattern in the deployed state of the device, additional to the primary scaffolding by the winding(s) of primary undulations. By placing the connection elements and the connection struts at regular intervals with respect to each other, this secondary scaffolding will consist of a continuous helical winding advancing in between the primary undulations. As a result the structure features an enhanced hoop strength and an improved stent-to-vessel ratio in its deployed state, which renders the device particularly suitable in peripheral vessels and arteries where it is likely to be subjected incidentally to considerable external loads.
In order to allow a large radial expandability as well as expanded flexibility, a special embodiment of the endoprosthesis according to the invention is characterized in that the connection strut comprises a sub-structure of mutually staggered secondary undulations, at least in a crimped state of the device, lying in between subsequent turns of primary undulations. Accordingly the sub-structure formed by these undulations may be stretched so as to give way to the radial expansion of the device as a whole. Moreover, the stretching capability of the secondary undulations offers an improved flexibility of the device both in a crimped as well as in a deployed state. The latter feature contributes to an excellent capability of the device to conform itself to the natural anatomy of the body lumen in which it is to be implanted.
A further special embodiment of the endoprosthesis according to the invention is characterized in that the secondary undulations have an amplitude substantial equal to half the longitudinal pitch of said turns. In this manner the second undulations bridge the gap between both windings of primary undulations at least to the largest possible extent, which gives rise to an optimal stent-to-vessel ratio in the expanded state.
Still a further special embodiment of the endoprosthesis according to the invention is characterized in said primary undulations have a mutual pitch which is at least locally substantially an integer multiple of the mutual pitch of said secondary undulations. Thus it is possible to arrange the secondary undulations in phase with the primary undulations in order to obtain a regular structure. This structural regularity translates to highly predictable expansion characteristics.
A further special embodiment of the endoprosthesis according to the invention is characterized in that the connection elements comprise at least one tertiary undulation which lies embedded in said substructure of mutually staggered secondary undulations of said connection struts, at least in said first, unexpanded state of the device. Like the secondary undulations, said tertiary undulation provides for an enhanced radial expandability of the device, By embedding the tertiary undulation in the same substructure which is formed by the second undulations a substantially continuous secondary scaffolding structure is realized in the device, interposed between the primary undulations.
A further embodiment of the endoprosthesis according to the invention is characterized in that at least a part of said wall of said tubular member comprises at least two substantially continuous windings of mutually staggered primary undulations, advancing mutually substantial parallel along the longitudinal axis of said tubular member, in that a first winding comprises said first and third undulation, in that a second winding comprises said second and fourth undulation, and in that the connection struts are interposed in between both said windings. This embodiment features a dual-helix structure provided by both said windings with interposed connection elements and connection struts. Upon expansion of this device the interposed substructure will re-orientate itself with respect to the longitudinal axis of the device such that the connection elements will turn substantially traverse to said axis whereas the connection struts will assume a more parallel orientation with respect to said axis. As a result pairs of connection elements may form discrete ring like elements substantially traverse to the axis of the body interconnecting both windings, in the deployed state of the device. These ring like elements are themselves interconnected by substantially longitudinally orientated connection struts. These bonds together give rise to excellent scaffolding properties and stent-to-vessel ratio of the device once it has been deformed to an expanded state, whereas the structure remains extremely flexible both in a crimped as well as in an expanded state. The total structure as a result features an outstanding hoop strength and stent-to-vessel ratio in the deployed state without compromising its crimped and deployed flexibility.
In a further preferred embodiment the endoprosthesis according to the invention is characterized in that said structure comprises a substantial continuous filament which has been separated from a tube wall, in that said connection elements are an integral extension of the primary undulations which they interconnect and in that the connection strut is an integral extension of the connection elements thereby connected. The filament may be formed by applying computer controlled laser cutting or another high precision technique to a solid tube. Thus the device may be formed in any configuration without introducing any stress in the structure.